1. Field of the Invention
The present invention relates to variable capacity wobble plate compressors having a compression capacity regulated by the variable angle of the wobble plate.
2. Description of the Prior Art
Variable capacity wobble plate compressors, the piston stroke of which is changed by adjusting the angle of the wobble plate to regulate the compression capacity, are well known. In such compressors, the angle of the wobble plate with respect to the drive shaft changes as the drive shaft rotates. This wobble movement causes the piston to reciprocate so that the refrigerant is compressed and expanded within the associated cylinder. A control valve is placed in a passage between the crank chamber for housing the wobble plate and the suction chamber to control the degree of opening of the passage and thus the pressure of the crank chamber. This adjusts the angle of the wobble plate and thus the piston stroke, thereby regulating the compression capacity.
As FIG. 9 shows, however, such variable capacity wobble plate compressors have an unstable (surging) zone U defined by a characteristic curve of the discharge pressure Pd (kg/cm.sup.2 G) as a function of the rotary speed (rpm) of the compressor. The hatched unstable area, where both the discharge pressure and the suction pressure become unstable, varies with the heat load (evaporator load) and the exciting current through the pressure regulating valve as described below.
For example, three characteristic curves are given in FIG. 10 for three different heat loads A, B, and C (A&gt;B&gt;C) while five characteristic curves are given in FIG. 11 for five different exciting currents D, E, F, G, and H (D&lt;E&lt;F&lt;G&lt;H). The characteristic curve D indicates that the piston has a full stroke at zero ampere. In these unstable zones, abnormal vibrations take place, making it impossible to control the angle of a wobble plate, and continuous operation can damage the compressor.